Testing systemic fungicides for control of Phytophthora oak root disease

Potassium phosphite (PP) formulations registered as fertilizers are now prohibited in Spain. Therefore, we evaluated the systemic fungicide fosetyl‐aluminium (fos‐al) in comparison with PP, against root rot caused by Phytophthora cinnamomi in Quercus woodlands. The direct effect of both systemic fungicides was evaluated in vitro on P. cinnamomi mycelial growth. Protection of cork and holm oak against infection was also evaluated in planta. Metalaxyl was included in both in vitro and in planta experiments for comparison purposes. At 100 μg/mL, PP totally inhibited colony radial growth, in comparison with 75% achieved by fos‐al. At doses recommended by manufacturers, with fos‐al and metalaxyl applications, root symptoms remained similar to the uninfected control levels. Based on these results, fos‐al is a candidate substitute product for PP in Quercus woodlands for control of Phytophthora oak root disease.     

Land‐use changes influence the sporulation and survival of Phytophthora agathidicida,a lethal pathogen of New Zealand kauri (Agathis australis)

Phytophthora agathidicida is the accepted causal agent of dieback in remnant stands of long‐lived indigenous New Zealand kauri (Agathis australis) and poses a significant threat to the long‐term survival of this species. Little is known about the effect of key soil physicochemical characteristics on the growth of P. agathidicida. In this study, we investigated the growth of P. agathidicida in soils collected from adjacent areas under original kauri forest, short rotation pine (Pinus radiata) plantation forest and grazed pastures. A growth response assay was used to quantify asexual (sporangia) and sexual (oospore) spore counts over 8 days in soils sampled from each land‐use. Significantly higher numbers of sporangia (p < 0.001) and oospores (p < 0.01) were found in pasture and pine forest soil within 2 days of the growth assay trials, suggesting these soils may favour asexual/sexual reproduction in the early stages of P. agathidicida establishment compared to kauri forest soils. Additionally, oospore production significantly increased over 8 days in pine forest soil, suggesting that with an increase in inoculum loads, these soils potentially act as pathogen reservoirs. The soil physicochemical properties (e.g., pH, C and N, phosphorus content and electrical conductivity) investigated in this study did not significantly correspond to spore count data between land‐uses, suggesting that differences in growth response are driven by other edaphic factors not explored in the present study.     

The long‐term survival of Phytophthora cinnamomi in mature Banksia grandis killed by the pathogen

The ability of Phytophthora cinnamomi to survive long dry periods is the key to its persistence in the south‐west of Western Australia. It has been proposed that dead Banksia grandis are a significant long‐term reservoir for P. cinnamomi inoculum. To test this, 36 healthy B. grandis trees were inoculated in April 1999, and the presence of viable propagules in planta was determined between 2 and 34 months after tree death. By 10 months after inoculation, 75% of the trees had died, with the remaining seven trees dying by 22 months. The pathogen was more commonly recovered from bark than from wood, except from those trees that died at 22 months, and more commonly from above‐ground trunks than below‐ground trunks and roots until 8 months after plant death. In trees that died 12 months after inoculation, P. cinnamomi was recovered from 60% of trunk and root core samples at 3 months, declining to 33% at 10 months, 5.5% at 12 months and 0.1% at 34 months after tree death. In trees that died at 22 months, P. cinnamomi was recovered from 87% of trunk and root samples 2 months after tree death, decreasing to 0.5% by 33 months. This study suggests that the pathogen does not have a saprotrophic phase within dead B. grandis tissue, and B. grandis is unlikely to be a long‐term reservoir for P. cinnamomi. However, the manipulation of the density of B. grandis and the use of fire to facilitate the breakdown of dead Banksia trunks in the Eucalyptus marginata (jarrah) forest may reduce the spread and impact of P. cinnamomi.     

Cross‐infectivity of oil palm by Phytophthora spp. isolated from perennial crops in Malaysia

Bud rot disease or “Pudricion del cogollo” (PC) of oil palm is a major constraint on production in Colombia and neighbouring countries such as Brazil, Costa Rica, Ecuador, Nicaragua, Panama, Peru and Surinam. To date, there are no documented reports of Phytophthora disease of oil palm in South‐East Asia. This research, therefore, was conducted to determine the pathogenic potential of Phytophthora palmivora and Phytophthora nicotianae on oil palm using both in vitro and nursery inoculation experiments. In vitro inoculation of both P. palmivora and P. nicotianae on immature oil palm leaflets caused discoloration within 2 days of inoculation and incubation at 25 ± 1.5°C, 100% RH. Similarly, in nursery trials, lesions formed on the buds (unopened leaflets) 3 days after inoculation with P. palmivora or P. nicotianae zoospore suspensions. No lesions developed on untreated leaflets in either in vitro or nursery inoculation experiments. Phytophthora spp. were re‐isolated from leaflet lesions and confirmed as the inoculated pathogens.     

Pathogenicity of Malaysian Phytophthora palmivora on cocoa,durian, rubber and oil palm determines the threat of bud rot disease

The epidemic of bud rot disease affecting oil palm in Colombia is primarily caused by Phytophthora palmivora. The pathogen has a cosmopolitan presence that includes Southeast Asia, but to date, bud rot has not been reported in this region. This study provides an overview of the potential risk of Malaysian P. palmivora isolates cross‐infecting other host species, including cocoa, durian, rubber and Malaysian oil palm planting materials (Dura × Pisifera, D × P). On cocoa pods, the durian isolate PP7 caused dark brown necrotic lesions. Detached leaf bioassays showed that P. palmivora isolates PP3 and PP7 infected different hosts, except rubber foliage without wounding. Inoculation tests on cocoa, durian and rubber seedlings caused brown necrotic lesions when stems were wounded, with 10% mortality in cocoa and durian at 17 days post‐inoculation (dpi). However, no further infection was observed, and lesions closed within 14–28 dpi on the non‐wounded seedlings. Pathogenicity tests of oil palm seedlings inoculated with isolates PP3 and PP7 indicated that Malaysian P. palmivora isolates were not pathogenic to oil palms based on localized infection observed only through wounding. Overall, the work demonstrated that Malaysian P. palmivora isolates were able to cross‐infect multiple hosts but did not show severe infections on oil palms.     

Temporal metabolic profiling of the Quercus suber–Phytophthora cinnamomi system by middle‐infrared spectroscopy

The oomycete Phytophthora cinnamomi is an aggressive plant pathogen, detrimental to many ecosystems including cork oak (Quercus suber) stands, and can inflict great losses in one of the greatest ‘hotspots’ for biodiversity in the world. Here, we applied Fourier transform‐infrared (FT‐IR) spectroscopy combined with chemometrics to disclose the metabolic patterns of cork oak roots and P. cinnamomi mycelium during the early hours of the interaction. As early as 2 h post‐inoculation (hpi), cork oak roots showed altered metabolic patterns with significant variations for regions associated with carbohydrate, glycoconjugate and lipid groups when compared to mock‐inoculated plants. These variations were further extended at 8 hpi. Surprisingly, at 16 hpi, the metabolic changes in inoculated and mock‐inoculated plants were similar, and at 24 hpi, the metabolic patterns of the regions mentioned above were inverted when compared to samples collected at 8 hpi. Principal component analysis of the FT‐IR spectra confirmed that the metabolic patterns of inoculated cork oak roots could be readily distinguished from those of mock‐inoculated plants at 2, 8 and 24 hpi, but not at 16 hpi. FT‐IR spectral analysis from mycelium of P. cinnamomi exposed to cork oak root exudates revealed contrasting variations for regions associated with protein groups at 16 and 24 h post‐exposure (hpe), whereas carbohydrate and glycoconjugate groups varied mainly at 24 hpe. Our results revealed early alterations in the metabolic patterns of the host plant when interacting with the biotrophic pathogen. In addition, the FT‐IR technique can be successfully applied to discriminate infected cork oak plants from mock‐inoculated plants, although these differences were dynamic with time. To a lesser extent, the metabolic patterns of P. cinnamomi were also altered when exposed to cork oak root exudates.     

A TaqMan real‐time PCR assay for the detection of Phytophthora ‘taxon Agathis’ in soil,pathogen of Kauri in New Zealand

Kauri Agathis australis, an iconic tree of New Zealand, is under threat from an introduced disease‐causing pathogen provisionally named Phytophthora ‘taxon Agathis’ (referred to as PTA). This soilborne, Pythiaceous species belongs to the Chromista and causes a collar rot resulting in yellowing of the foliage and thinning of the canopy, which eventually causes death of the infected tree. The management and containment of this pathogen requires rapid and reliable detection in the soil. The current method for soil detection utilizes a soil bioassay involving lupin baits and soil flooding in a process that takes between ten and twenty days. We describe a real‐time PCR assay based on TaqMan chemistry for the specific detection of PTA, which targets the internal transcribed spacer (ITS) region of the nuclear ribosomal DNA. This TaqMan real‐time PCR assay could be used with DNA extracted directly from bulk soil samples to enable rapid quantification of PTA within soil. The detection limit was 2 fg of PTA DNA from pure culture, or 20 fg in the presence of DNA extracted from soil. The assay was validated using soil samples taken from a PTA‐infested site and soil spiked with a known concentration of oospores. We conclude that the TaqMan real‐time PCR assay offers a more time‐efficient method for detection of PTA in soil than existing methods.     

The role of yellow lupin (Lupinus luteus) in the decline affecting oak agroforestry ecosystems

Phytophthora cinnamomi is a soilborne pathogen causing root rot in Mediterranean Quercus species growing in ‘dehesa’ rangeland ecosystems. Recently, it has been reported causing wilting and death of Lupinus luteus (yellow lupin), a spontaneous plant in southern Spain rangelands, but also frequently sowed for livestock grazing. In soils artificially infested with P. cinnamomi chlamydospores and planted with different cultivars of yellow lupin, a significant increase in the density of propagules was detected in comparison with the initial levels of inoculum and with the infested but not planted soil (control). In oak‐rangelands in which yellow lupine was planted, isolation and counting of colonies of P. cinnamomi from soil samples have shown the ability of this plant to maintain or even increase the inoculum density and thus facilitate the infection of trees. Results suggested that cultivation of yellow lupin in oak‐rangeland ecosystems should be avoided whether oak trees are affected by root disease caused by P. cinnamomi or not. This leguminous plant can act as an inoculum reservoir or even enhance inoculum soil levels available for oak root infections, exacerbating the oak decline severity in the region.     

Pathogenicity of Phytophthora species and Pythium undulatum isolated from Abies procera Christmas trees in Ireland

Phytophthora cryptogea, Phytophthora cinnamomi, Phytophthora cambivora, Phytophthora megasperma and Pythium undulatum were isolated from diseased Noble fir (Abies procera) seedlings and soil associated with dead Noble fir in Ireland. Seedlings of four Christmas tree species (A. procera, Picea sitchensis, Picea abies and Pinus contorta) were inoculated with these oomycetes to test their pathogenicity and the susceptibility of the various tree species. Phytophthora spp. and Pythium undulatum caused root rot on all tree species. Disease symptoms included reddish brown cambial discoloration, crown symptoms, brown foliage, dark brown roots, root rot and seedling mortality. These symptoms were similar to those observed on Noble fir in naturally infested plantations. Pythium undulatum appeared as the most virulent pathogen followed by P. cinnamomi, P. cambivora, P. megasperma and P. cryptogea. Noble fir showed to be most susceptible and lodgepole pine most tolerant while Sitka spruce and Norway spruce were intermediate.     

Fine root dynamics of oak saplings in response to Phytophthora cinnamomi infection under different temperatures and durations

The belowground effects of Phytophthora cinnamomi on 1‐year‐old saplings of two common oak species in mid‐Atlantic US forests, white (Quercus alba) and red oak (Q. rubra), were examined after incubation in pathogen‐infested soilless potting mix. Fine root lengths (0–1.5 mm in diameter) of both oak species were quantified after incubation at successive 30‐day intervals up to 300 days, for a total of 10 incubation periods. In addition, colony‐forming units (CFU) of P. cinnamomi were quantified after white oak saplings were incubated in infested soilless potting mix at different temperature/duration combinations that reflect soil conditions present in the mid‐Atlantic United States. Impact of P. cinnamomi on fine root lengths of red and white oak saplings varied considerably over time. Significant periods of fine root loss occurred primarily during spring, when bud break and leaf flush began for both oak species. Red oaks had 17% fine root loss on average, while white oaks appeared more resistant to P. cinnamomi infection with a 2% decrease in fine roots over the course of the experiment. Phytophthora cinnamomi CFU declined significantly with exposure to all incubation temperatures except 8°C. This was in contrast to in vitro experiments, where the optimum temperature for mycelial growth was determined to be 21°C and above. Significant fine root loss caused by P. cinnamomi depended on plant phenology and the oak species tested. Extreme soil temperatures have a significant adverse impact on temporal changes of P. cinnamomi population.     

New insights into the survival strategy of the invasive soilborne pathogen Phytophthora cinnamomi in different natural ecosystems in Western Australia

Despite its importance as one of the most notorious, globally distributed, multihost plant pathogens, knowledge on the survival strategy of Phytophthora cinnamomi in seasonally dry climates is limited. Soil and fine roots were collected from the rhizosphere of severely declining or recently dead specimens of 13 woody species at 11 dieback sites and two dieback spots and from healthy specimens of five woody species at four dieback‐free sites in native forests, woodlands and heathlands of the south‐west of Western Australia (WA). Phytophthora cinnamomi was recovered from 80.4, 78.1 and 100% of tested soil, fine root and soil–debris slurry samples at the 11 dieback sites, in some cases even after 18‐month storage under air‐dry conditions, but not from the small dieback spots and the healthy sites. Direct isolations from soil–debris slurry showed that P. cinnamomi colonies exclusively originated from fine roots and root fragments not from free propagules in the soil. Microscopic investigation of P. cinnamomi‐infected fine and small woody roots and root fragments demonstrated in 68.8, 81.3 and 93.8% of samples from nine woody species the presence of thick‐walled oospores, stromata‐like hyphal aggregations and intracellular hyphae encased by lignitubers, respectively, while thin‐walled putative chlamydospores were found in only 21.2% of samples from five woody species. These findings were confirmed by microscopic examination of fine roots from artificially inoculated young trees of 10 woody species. It is suggested that (i) the main function of chlamydospores is the survival in moderately dry conditions between consecutive rain events and (ii) selfed oospores, hyphal aggregations, and encased hyphae and vesicles in infected root tissue of both host and non‐host species are the major long‐term survival propagules of P. cinnamomi during the extremely dry summer conditions in WA.     

Involvement of Phytophthora spp. in chestnut decline in the Black Sea region of Turkey

Chestnut blight caused by Cryphonectria parasitica is a serious disease of Castanea sativa in the Black Sea region of Turkey. During disease surveys, dieback and decline symptoms were observed on trees without apparent blight and ink disease symptoms. Black necroses, similar to those caused by Phytophthora infections, were noted on some of the chestnut coppices and saplings in one nursery in Ordu and led to an investigation into this disease complex. Only symptomatic plants showing dieback symptoms were investigated. Soil samples together with fine roots were collected from two directions, north and north‐east, approximately 150 cm away from the main stems. Phytophthora spp. were baited with young chestnut leaves. Three Phytophthora spp., P. cambivora, P. cinnamomi and P. plurivora, were identified from 12 soil samples collected from 73 locations, while from the nurseries, only P. cinnamomi was obtained. Phytophthora cinnamomi was the most common species, obtained from seven locations in five provinces and from four nurseries having similar symptoms mentioned above in different locations. Phytophthora cambivora and P. plurivora were less frequently obtained, from three to two stands, respectively. Phytophthora cinnamomi and P. cambivora were the most aggressive species when inoculated at the stem base on 3‐year‐old chestnut saplings, killing six saplings of eight inoculated in 2 months. The three Phytophthora species were first recorded on chestnut in Black sea region of Turkey with the limited samples investigated in a large area about 150 000 ha chestnut forest.     

First report of Phytophthora pseudosyringae recovered from aquatic ecosystems in Bulgaria

Two Phytophthora pseudosyringae isolates were recovered from aquatic ecosystems in Bulgaria during a two‐year investigation of Phytophthora distribution in water sources in the country. Isolate RVit2016/6d was derived from Boyana Lake at the Vitosha Mountain, whereas isolate RTr2016/32d was obtained from Erma River at the Ruy Mountain. Both isolates belong to P. pseudosyringae species according to their morphological and physiological characteristics, as well as to the DNA sequence analysis of the internal transcribed spacer (ITS) region. The pathogenicity of the isolates to wild cranberry plants (Vaccinium vitis‐idea) was studied by detached leaves experiments and in planta. Both P. pseudosyringae isolates were able to cause leaf necrosis and death of plants within 3 months. The ability of the pathogens to infect cranberry leaves at different temperatures was also investigated. The significance of P. pseudosyringae species and its potential threat for forest ecosystems is discussed. To our knowledge, this is the first report of P. pseudosyringae isolation in Bulgaria.     

Pathogenic fungi associated with pre‐ and post‐emergence seedling blight of pine and cypress in Fars Province,Iran

Seed and root rot of pine and cypress seedlings cause heavy annual losses to forest nurseries in Fars Province. Root and crown samples of various species of conifers, such as Tehran pine (Pinus eldarica), brutia pine (Pinus brutia), Arizona cypress (Cupressus arizonica), Shirazian cypress (Cupressus sempervirens var. fastigiata), common cypress (C. sempervirens var. horizontalis) and oriental arborvitae (Thuja orientalis), showing chlorosis, necrosis, stunted growth, defoliation and root and crown rot symptoms were collected from pine and cypress nurseries across Fars province at various time intervals. Infected tissues were washed and cultured on acidified potato dextrose agar (PDA) and corn meal agar (CMA) amended with Delvocide and ampicillin, with and without surface sterilization, respectively, and incubated at 25°C for 3–5 days. Decaying seeds of all plant species were also collected from nursery seed stocks and cultured on PDA. Hyphal tip isolates were used for further studies. Species of Phytophthora, Pythium, Rhizoctonia and Fusarium were isolated from symptomatic seedlings of pine and cypress at different times during the growing season. Pathogenicity of isolates of Phytophthora, Pythium and Rhizoctonia was confirmed on seedlings of all plant species, whereas Fusarium sp. isolated from the seeds of Tehran pine was only pathogenic on seedlings of Tehran pine and Arizona cypress. Virulent isolates identified as Phytophthora nicotiana var. parasitica, Pythium ultimum, Pythium paroecandrum, Rhizoctonia solani and binucleate Rhizoctonia‐like fungus caused root and crown rot of pine and cypress seedlings. Pathogenic isolates of Fusarium identified as Fusarium proliferatum also caused pre‐emergence seed decay of pine and cypress. Isolates of Phytophthora and Pythium were the most virulent of these fungal isolates.     

Isolation of Phytophthora lateralis from Chamaecyparis foliage in Taiwan

Following the discovery in 2008 of Phytophthora lateralis in forest soil under old‐growth yellow cedar (Chamaecyparis obtusa var. formosana) in north‐east Taiwan, further sampling was undertaken in the same region. Soil, root and symptomatic foliage samples were collected from five separate sites where C. obtusa was the dominant species in cloud forests at ca. 1800–2500 m. Soil and fine root samples were baited with cedar needles; both direct isolation and cedar needle baiting were used on foliage samples. Phytophthora lateralis was obtained from soil at three of the sites, but only from three of the 27 soil samples overall. Only one of 25 root samples yielded the pathogen, and this was associated with infested soil. Three foliage samples with symptoms visible as dark brown to black frond tips also yielded P. lateralis; these came from two different sites. This is the first record of P. lateralis infecting the foliage of C. obtusa. Moreover, when some of the symptomatic Chamaecyparis foliage segments were incubated, sporangia of P. lateralis formed on the necrotic tissues, sometimes in the axils of needle segments. The study provides evidence that P. lateralis has both a soil/root infecting phase and an aerial or foliar infecting phase in Taiwan, which is consistent with its unusual combination of water‐dispersed (non‐papillate) and aerially dispersed (caducous) sporangia. It also demonstrates the importance of investigating the biology, aetiology and ecological behaviour of Phytophthoras in their native, endemic environments.     

S and P intersterility groups in Heterobasidion annosum; Infection frequencies through bark of Picea abies and Pinus sylvestris seedlings and in vitro growth rates at different oxygen levels

Seedlings of Norway Spruce and Scots Pine were inoculated with S and P strains of the root rot fungus Heterobasidion annosum. Infection frequencies through the living bark layers were not related to subgroup affiliation in neither spruces nor pines. Growth rates in vitro were not related to growth rates in vivo. Oxygen concentration had a stronger effect on the growth rates of P than on S strains.     

Pathogenicity of three Phytophthora spp. causing late seedling rot of Quercus ilex ssp. ballota

Within a research project on quality of plants for forestation of agricultural lands, we studied the aetiology of a late seedling rot affecting holm oak (Quercus ilex ssp. ballota) in two forest nurseries in southern Spain. Major disease symptoms were foliage wilting and necrosis of feeder roots. Phytophthora cinnamomi, Phytophthora cryptogea and Phytophthora drechsleri were isolated from necrotic roots of holm oaks. Selected isolates of the three Phytophthora species were pathogenic to Quercus ilex ssp. ballota and Quercus suber seedlings in artificial inoculations. Soil flooding conditions were essential for infection and root rot development. There was no host specificity among the species, the isolates of P. cinnamomi being the most virulent in all inoculated plants. In these inoculations, Q. ilex ssp. ballota plants were more susceptible than those of Q. suber. This work is the first report of P. cinnamomi, P. drechsleri and P. cryptogea affecting Q. ilex ssp. ballota in forest nurseries.     

Susceptibility of Eucalyptus grandis and Acacia mearnsii seedlings to five Phytophthora species common in South African plantations

Eucalyptus grandis and its hybrids, as well as Acacia mearnsii, are important non‐native trees commonly propagated for forestry purposes in South Africa. In this study, we conducted pathogenicity trials to assess the relative importance of five commonly isolated Phytophthora spp. (Phytophthora alticola, P. cinnamomi, P. frigida, P. multivora and P. nicotianae) from the plantation environment on E. grandis and A. mearnsii seedlings. Overall E. grandis was more susceptible to the tested Phytophthora spp. than A. mearnsii. Phytophthora cinnamomi was the only pathogen that had a significant negative effect on both the host tree species, leading to a reduction in root and shoot weight as well as to death in the case of E. grandis. Phytophthora alticola and P. nicotianae exclusively affected E. grandis and A. mearnsii, respectively. This study updated the current knowledge on the pathogenicity of Phytophthora spp. on two important non‐native commercially propagated tree species from South Africa.     

Pathogenicity to alder of Phytophthora species from riparian ecosystems in western Oregon

Described as one of the most destructive pathogens of agricultural crops and forest trees, Phytophthora is a genus of microorganisms containing over 100 known species. Phytophthora alni has caused collar and root disease in alders throughout Europe, and a subspecies has recently been isolated in North America. Reports of canopy dieback in red alder, Alnus rubra, prompted a survey of their overall health and to determine whether P. alni was present in western Oregon riparian ecosystems. Over 1100 Phytophthora isolates were recovered, representing 20 species and 2 taxa. Phytophthora‐type cankers were observed in many trees, and their incidence was positively correlated with canopy dieback. High levels of mortality for red alder were not observed, which suggests these Phytophthora species are not aggressive pathogens. To test this hypothesis, three stem wound inoculations and one root dip were conducted on red alder seedlings using 13 Phytophthora species recovered from the riparian survey. Ten of the 13 Phytophthora species produced significant lesions in at least one pathogenicity test. Phytophthora siskiyouensis produced the largest lesions on red alder from the two stem wound inoculation tests conducted under summer conditions, while P. taxon Pgchlamydo caused the largest lesions during the winter stem wound inoculation test. Phytophthora gonapodyides, P. taxon Pgchlamydo and P. siskiyouensis have previously been found associated with necrotic alder roots and bole cankers in the field, and with the pathogenicity results reported here, we have established these species as causes of Phytophthora root disease and Phytophthora bole canker of alder in Oregon. While none of the Phytophthora species were especially aggressive towards red alder in the pathogenicity tests, they did cause localized disease symptoms. By weakening the root systems or boles of alders, the Phytophthoras could be leaving alders more susceptible to other insects and pathogens.